Mercury vapor tube



Spt. 23, 1947. R. c. MASON MERCURY VAPOR TUBE Filed Aug. 6, 1941 cur-ve for 'Ue F/g Z.'

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ATTORNE Patented Sept. 23, 1947 UNITED STATES PATENT OFFICE MERCURY VAPOR TUBE Application August 6, 1941, Serial No. 405,583

(Cl. Z50-27.5)

3 Claims.

My invention relates to electrical discharge devices and, in particular, to electrical discharge devices embodying an atmosphere of mercury or other vapor.

One object of my invention is to produce a mercury or other vapor electrical discharge device in which the electrical characteristics of the tube are less sensitive to changes in ambient temperature than are those of tubes of the prior art.

Another object of my invention is to provide a mercury or other vapor electrical discharge device which can operate at a relatively low vapor pressure and yet be less affected by electrical clean-up than are electrical discharge devices of the prior art operating at similar pressures.

Other objects of my invention will become apparent upon reading the following specification taken in connection with the drawing, in which:

Figure 1 shows a curve connecting vapor pressure and ambient temperature which will be used in the following explanation of my invention;

Fig. 2 shows an electrical discharge device embodying the principles of my invention; and,

Fig. 3 and Fig. 4 show forms of an electrical discharge device alternative to Fig. 2.

Electrical discharge devices of the prior art having thermionically-emissive cathodes, anodes, and frequently having control electrodes, which operate in an atmosphere of mercury or other vapor which is in equilibrium with its liquid at the temperature of some portion of the tube walls, are well known. It is found that such discharge tubes are unsatisfactorily sensitive to changes of the ambient temperature in which the tube operates, or to gradual heating up of the tube walls during operation, by reason of the comparatively rapid variation of vapor pressure with temperature of the liquid. For example, the curve I shows the variation of the pressure P with temperature T of mercury vapor which is in equilibrium with liquid mercury. Thus at temperatures above that corresponding to the ordinate marked a, the pressure rises very rapidly with comparatively small variations of temperature, vand the rate of this rise increases rapidly as the temperature rises further. The electrical characteristics, such as the curve connecting the anode voltage at which the arc ignites with control-electrode voltagey are materially affected by variations of mercury pressure within the tube. Any rise in the ambient temperature about the tube, or even the heating up of the tube walls during operation, causes a rise in the temperature T of the liquid mercury within the tube, and this' results in an increase of the pressure Within the tube in corre- 2 spondence with the curve I. As a result, the electrical characteristics of the tube are quite sensitive to variations of the ambient temperature.

I have found that mercury vapor may be adsorbed by a quantity of so-called activated carbon, that is to say, carbon prepared in a form in which carbon grains are interspersed with a multitude of fine holes or cavities; and that this adsorption results in an alteration of the curve connecting vapor pressure P of the mercury with ambient temperature or temperature T of the container. This is illustrated by the curves II and III in Fig. 1. The curve II represents the variation of pressure and temperature for carbon which has adsorbed a certain number q1 of atoms of mercury per gram of carbon, and the curve III represents the variation of pressure with temperature for the same carbon in which a number q2 of atoms of mercury are adsorbed per gram of carbon, q2 being greater than q1. The curve II may, in fact, be represented by the equation:

where q1 has the constant value above mentioned, T the temperature in degrees Kelvin, and e the base of Napierian logarithms.

Curve III would be represented by a similar equation in which q2 replaces q1. Such curves may be called isosteres.

It will be noticed that each of the curves II and III intersects the curve I at a certain point; below this point the curve of vapor pressure P is represented by the curve I and at higher temperatures by the curve II or III, as the case may be. It will be further noticed that the rise of pressure with temperature along the curve II is, in general, much less than that along the curve I, which latter represents the pressure variation in a tube containing no activated carbon. The same is true, in general, of the curve III. It thus follows that by including activated carbon in the tube containing mercury, the vapor pressure P and electrical characteristics of the latter will be much less aITected by variations of the ambient temperature T than will tubes which contain no activated carbon.

Methods of making activated carbon are well known in the chemical art, so that their detailed description is not necessary here. It is my belief that, in general, the adsorbing power of the carbon for mercury vapor increases as the exposed surface (including the exposed surface within small cavities) is increased for a given weight of carbon. One familiar form of acti- 3 vated carbon is charcoal, that made by properly carbonizing cocoanut shells being frequently used.

Since the reduced vapor pressure due to the presence of activated carbon in the tube is obtained only at temperatures higher than that corresponding to the intersection of the isostere with the curve I, it is desirable that this intersection shall occur at temperatures below any room temperature likely to be met in practice, or at least below the temperature attained by the walls of the tube in normal operation. The temperature of this intersection point can be varied by varying the quantity f mercury which the activated carbon is permitted to adsorb before it is placed in the tube. The Equation l above may be combined with the two following equations to calculate the number of mercury atoms to be adsorbed in the carbon, and also to calculate a theoretical curve connecting pressure P within the tube with ambient or wall temperature T.

A certain number of mercury atoms, will, in general, be present in the tube when it .is sealed oi after evacuation, and these will obviously be distributed partly as a vapor filling the evacuated space in the tube and partly as mercury atoms adsorbed in the activated carbon. The number of mercury atoms present within the evacuated volume of the tube may be calculated .from the equation:

(2) N=3.55 1016PV where V is the volume of the tube in cubic centimeters.

The number of atoms of mercury adsorbed in the activated carbon will be where w is the weight of the carbon present in grams, and q is as in Equation 1 above.

Since the number of atoms of mercury in the tube is the sum of those present as a gas in the volume V and those adsorbed in the w grams of carbon, and this quantity is always constant once the tube has been sealed 01T, it is possible to write the following equation:

By substituting the value of q given by Equation 1 in Equation 3 there is obviously obtained an equation connecting a pressure P with temperature T which will represent the variation of pressure within the tube and its ambient temperature.

The foregoing equations may be used to design an electrical discharge tube in the following way. When the pressure in a tube during operation rises to too high a value, the control grid loses its effectiveness in controlling mean current flow. Thus the pressure Pu in the tube at its highest expected operating temperature Tu is decided upon in advance by the designer. The lowest temperature Ta at which the tube will have to operate will also be known to the designer, and the isostere should intersect the curve I at a temperature corresponding to Ta; and this intersection likewise determines the corresponding pressure Pa. By substituting this pressure Pa and the temperature Ta in Equation 1, the number qu. of mercury atoms to be adsorbed per gram of activated carbon at the lowest operating temperature Ts is given. The number qu of mercury atoms to be adsorbed per gram of activated carbon at the highest temperature Tu and pressure Pu are likewise determinable by subqw+N=constant 4 stitution in Equation 1. The volume V of the evacuated space within the tube may be arbitrarily xed, but once this has been done, this volume V may be substituted with the pressures Pa and Pu just determined from the curve I in Equation 2, and the number of mercury atoms Ns and Nu present within the evacuated space of the tube at the temperatures Ta and Tu may be determined. By substituting qa, Nn, qu, Nu in Equation 4 there results an equation (5) qaw-i-Na=constant=quw+Nu which may be solved for w, the number of grams of activated charcoal to be enclosed in the tube. Physical-chemical tables give the number of atoms of mercury per gram of mercury, so that the required quantity of mercury (qaw-l-Na) can thus be calculated and weighed out. The dcsign of the tube, insofar as it relates to the present invention, has thus been determined by the foregoing procedure. Curve IV in Fig. 1 represents the relationship between pressure P and ambient temperature T of a tube, as given by Equation 4.

By using activated carbon the amount of mercury adsorbed just about the intersection of the curve I with that obtained from Equation 3 is large compared to the amount of mercury in the gas phase represented by Equation 2, and this insures that any clean-up or disappearance of mercury atoms due to continued use of the tube after manufacture will have only a small effect on the vapor pressure within the tube. In the prior art the attempt has occasionally been made to avoid the rapid pressure-rise characteristic of curve I by using tubes in which no liquid mercury was present. But in these tubes clean-up of mercury vapor during the life of the tube has resulted in troublesome variation of the gaseous pressure.

While I have referred to the particular vapor present in the tube as being mercury, it will be recognized that this is only for purposes of illustration. Other vapors are subject to adsorption by activated carbon and their substitution for mercury is within the purview of my invention. The principles which I have illustrated by the foregoing equations apply to vapors generally. It will also be recognized that while I have mentioned activated carbon, there are other substances capable of carrying out the adsorbing effect and these may be substituted when otherwise desirable for the activated carbon I have mentioned.

To illustrate the form of actual tubes capable of embodying my invention, Fig. 2 shows a container I which may be of glass or other suitable material and capable of maintaining a vacuum. An anode 2 and a thermionically-emissive cathode 3 of conventional form may be embodied in the tube in ways Well known in the electrical discharge tube art. Likewise, a control electrode 4 may be provided.

The annular space surrounding the press through which the cathode and grid leads are sealed may be used as a receptacle for the activated carbon mentioned above.

Figs. 3 and 4 illustrate electrical discharge tubes which are like that shown in Fig. 2, except that each is provided with a. side branch or sidepocket 5 in which the activated carbon is contained.

While I have illustrated particular embodiments ofv my invention, it will be recognized by those skilled in the art that its principles are of broad application and may be practiced with many different forms of tube.

I claim as my invention:

1. An electrical discharge device comprising a vacuum-tight container having electrodes therein and containing Within its Walls activated carbon within which is adsorbed a substantial quantity of mercury, the number of atoms of mercury in the tube being less than Where T is the normal operating temperature of the Walls of said discharge device in degrees Kelvin, P is the pressure in millimeters of mercury which corresponde to on vthe vapor-pressure v. temperature curve of mercury, e is the base of Napierian logarithms, w is the number of grams of activated carbon in the container and V is the volume of the container in cubic centimeters.

2. An electrical discharge device comprising a vacuum-tight container having an anode, an electrcn-emissive cathode and a control electrode therein and containing within its Walls activated carbon within Which is adsorbed a substantial quantity of mercury, the number of atoms of mercury in the tube being less than 15 w @To-w56 2T +3.55 1o16Pv where T is the normal operating temperature of the Walls of said discharge device in degrees Kelvin, P is the pressure in millimeters of mercury which corresponds to T on the vapor-pressure v. temperature curve of mercury, e is the base of Napierian logarithme, w is the number of grams of activated carbon in the container, and V is the volume of the container in cubic centimeters.

3. An electrical discharge device comprising a vacuum-tight container having electrodes therein and containing Within its Walls activated carbon Within which is adsorbed a substantial quantity of mercury, the number of atoms of mercury in the tube being about REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,041,610 Killian May 19, 1936 1,125,476 Claude Jan, 19, 1915 1,841,034 Ives Jan. 12, 1932 FOREIGN PATENTS Number Country Date 230,467 Great Britain June 4, 1926 240,863 Great Britain Apr. 4, 1927 

